[bug #51072] groff.texi: "Diversions" material from groff_tmac(5) missing

[G. Branden Robinson]

Update of bug #51072 (project groff):

                  Status:                    None => Need Info              
             Assigned to:                    None => gbranden               
                 Summary: groff.info: Tips on writing a working diversion is
missing => groff.texi: "Diversions" material from groff_tmac(5) missing

    _______________________________________________________

Follow-up Comment #1:

[comment #0 original submission:]
>   As the "info" file is claimed to be the best documentation about "groff",

I don't think any of our documentation makes that claim anymore.

If it does, please point it out to me.

> a text about writing a diversion that works is missing from it.

The node/section on diversions has been moderately revised since groff
1.22.4.


$ git diff -U100 1.22.4 doc/groff.texi | sed -n '/^.@node
Diversions/,/^.@node/p'
 @node Diversions, Environments, Traps, gtroff Reference
 @section Diversions
 @cindex diversions
 
-In @code{gtroff} it is possible to @dfn{divert} text into a named
-storage area.  Due to the similarity to defining macros it is sometimes
-said to be stored in a macro.  This is used for saving text for output
-at a later time, which is useful for keeping blocks of text on the same
-page, footnotes, tables of contents, and indices.
-
+In @code{roff} systems it is possible to format text as if for output,
+but instead of writing it immediately, one can @dfn{divert} the
+formatted text into a named storage area.  The same name space is used
+for such @emph{diversions} as for strings and macros;
+@xref{Identifiers}.  Such text is sometimes said to be ``stored in a
+macro'', but this coinage obscures the important distinction between
+macros and strings on one hand and diversions on the other; the former
+store @emph{unformatted} input text, and the latter capture
+@emph{formatted} output.  Applications of diversions include ``keeps''
+(preventing a page break from occurring at an inconvenient place by
+forcing a set of output lines to be set as a group), footnotes, tables
+of contents, and indices.
 @cindex top-level diversion
 @cindex diversion, top-level
-For orthogonality it is said that @code{gtroff} is in the @dfn{top-level
-diversion} if no diversion is active (i.e., the data is diverted to the
-output device).
-
-Although the following requests can be used to create diversions,
-simply using an undefined diversion will cause it to be defined as empty.
-@xref{Identifiers}.
-
-@DefreqList {di, macro}
-@DefreqListEndx {da, macro}
+For orthogonality it is said that GNU @code{troff} is in the
+@dfn{top-level diversion} if no diversion is active (that is, formatted
+output is being ``diverted'' immediately to the output device).
+
+Dereferencing an undefined diversion will create an empty one of that
+name and cause a warning of type @samp{mac} to be emitted
+(@pxref{Debugging}).  A diversion does not exist for the purpose of
+testing with the @code{d} conditional operator until its initial
+definition ends (@pxref{Operators in Conditionals}).  The following
+requests are used to create and alter diversions.
+
+@DefreqList {di, [@Var{name}]}
+@DefreqListEndx {da, [@Var{name}]}
 @cindex beginning diversion (@code{di})
 @cindex diversion, beginning (@code{di})
 @cindex ending diversion (@code{di})
 @cindex diversion, ending (@code{di})
 @cindex appending to a diversion (@code{da})
 @cindex diversion, appending (@code{da})
-Begin a diversion.  Like the @code{de} request, it takes an argument of
-a macro name to divert subsequent text into.  The @code{da} macro
-appends to an existing diversion.
+Start collecting formatted output in a diversion called @var{name}.  The
+@code{da} request appends to the existing diversion called @var{name},
+creating it if necessary.  The pending output line is diverted as well.
+Switching to another (empty) environment (with the @code{ev} request)
+avoids the inclusion of the current partially collected line;
+@ref{Environments}.
 
-@code{di} or @code{da} without an argument ends the diversion.
+Invoking @code{di} or @code{da} without an argument stops diverting
+output to the diversion named by the most recent corresponding request.
+If @code{di} or @code{da} is called without an argument when there is no
+current diversion, a warning of type @samp{di} is produced.
+@xref{Debugging}, for information about the enablement and suppression
+of warnings.
 
-The current partially filled line is included into the diversion.  See
-the @code{box} request below for an example.  Note that switching to
-another (empty) environment (with the @code{ev} request) avoids the
-inclusion of the current partially filled line.
+@Example
+Before the diversion.
+.di yyy
+In the diversion.
+.br
+.di
+After the diversion.
+.br
+    @result{} After the diversion.
+.yyy
+    @result{} Before the diversion.  In the diversion.
+@endExample
 @endDefreq
 
-@DefreqList {box, macro}
-@DefreqListEndx {boxa, macro}
-Begin (or append to) a diversion like the @code{di} and @code{da}
-requests.  The difference is that @code{box} and @code{boxa} do not
-include a partially filled line in the diversion.
+@cindex box (diversion type)
+Because it is often desirable to exclude the partially collected line
+from a diversion, @code{groff} supports an alternative form of diversion
+known as a @dfn{box}.
 
-Compare this:
+@DefreqList {box, [@Var{name}]}
+@DefreqListEndx {boxa, [@Var{name}]}
+Divert (or append) output to @var{name}, similarly to the @code{di} and
+@code{da} requests, respectively.  Any output line pending when these
+requests are invoked is @emph{not} included in the box.  Calling
+@code{box} or @code{boxa} without an argument stops diverting output to
+the box named by the most recent corresponding request; a pending output
+line inside a diversion is discarded.
 
 @Example
 Before the box.
 .box xxx
 In the box.
 .br
+Hidden treasure.
 .box
 After the box.
 .br
     @result{} Before the box.  After the box.
 .xxx
     @result{} In the box.
 @endExample
-
-@noindent
-with this:
-
-@Example
-Before the diversion.
-.di yyy
-In the diversion.
-.br
-.di
-After the diversion.
-.br
-    @result{} After the diversion.
-.yyy
-    @result{} Before the diversion.  In the diversion.
-@endExample
-
-@code{box} or @code{boxa} without an argument ends the diversion.
 @endDefreq
 
+Apart from pending output line inclusion and the request names that
+populate them, boxes are handled exactly as diversions are.  All of the
+following @code{groff} language elements can be used with them
+interchangeably.
+
 @DefregList {.z}
 @DefregListEndx {.d}
 @cindex @code{nl} register, and @code{.d}
 @cindex nested diversions
 @cindex diversion, nested
 @cindex diversion name register (@code{.z})
 @cindex vertical position in diversion register (@code{.d})
 @cindex position, vertical, in diversion, register (@code{.d})
 @cindex diversion, vertical position in, register (@code{.d})
-Diversions may be nested.  The read-only number register @code{.z}
-contains the name of the current diversion (this is a string-valued
-register).  The read-only number register @code{.d} contains the current
-vertical place in the diversion.  If not in a diversion it is the same
-as register @code{nl}.
+Diversions may be nested.  The read-only string-valued register
+@code{.z} contains the name of the current diversion.  The read-only
+register @code{.d} contains the current vertical place in the diversion.
+If the input text is not being diverted, @code{.d} reports the same
+location as the register @code{nl}.
 @endDefreg
 
 @Defreg {.h}
 @cindex high-water mark register (@code{.h})
 @cindex mark, high-water, register (@code{.h})
 @cindex position of lowest text line (@code{.h})
 @cindex text line, position of lowest (@code{.h})
-The @dfn{high-water mark} on the current page or in the current
-diversion.  It corresponds to the text baseline of the lowest line
-on the page.  This is a read-only register.
+The read-only register @code{.h} stores the @dfn{high-water mark} on the
+current page or in the current diversion.  It corresponds to the text
+baseline of the lowest line on the page.@footnote{Thus, the ``water''
+gets ``higher'' proceeding @emph{down} the page.}
 
 @Example
 .tm .h==\n[.h], nl==\n[nl]
     @result{} .h==0, nl==-1
 This is a test.
 .br
 .sp 2
 .tm .h==\n[.h], nl==\n[nl]
     @result{} .h==40, nl==120
 @endExample
 
 @cindex @code{.h} register, difference to @code{nl}
 @cindex @code{nl} register, difference to @code{.h}
 @noindent
-As can be seen in the previous example, empty lines are not considered
-in the return value of the @code{.h} register.
+As implied by the example, vertical spacing does not produce text
+baselines and thus does not increase the value interpolated by
+@samp{\n[.h]}.
 @endDefreg
 
 @DefregList {dn}
 @DefregListEndx {dl}
 @cindex @code{dn} register, and @code{da} (@code{boxa})
 @cindex @code{dl} register, and @code{da} (@code{boxa})
 @cindex @code{da} request, and @code{dn} (@code{dl})
 @cindex @code{boxa} request, and @code{dn} (@code{dl})
-After completing a diversion, the read-write number registers @code{dn}
-and @code{dl} contain the vertical and horizontal size of the diversion.
-Note that only the just processed lines are counted: For the computation
-of @code{dn} and @code{dl}, the requests @code{da} and @code{boxa} are
-handled as if @code{di} and @code{box} had been used -- lines that have
-been already stored in a macro are not taken into account.
+After completing a diversion, the writable registers @code{dn} and
+@code{dl} contain the vertical and horizontal size of the diversion.
+Only the lines just processed are counted: for the computation of
+@code{dn} and @code{dl}, the requests @code{da} and @code{boxa} are
+handled as if @code{di} and @code{box} had been used,
+respectively---lines that have been already stored in the diversion
+(box) are not taken into account.
 
 @Example
-.\" Center text both horizontally & vertically
+.\" Center text both horizontally and vertically.
 .
-.\" Enclose macro definitions in .eo and .ec
-.\" to avoid the doubling of the backslash
+.\" Disable the escape character with .eo so that we
+.\" don't have to double backslashes on the "\n"s.
 .eo
-.\" macro .(c starts centering mode
+.\" Macro .(c starts centering mode.
 .de (c
 .  br
 .  ev (c
 .  evc 0
 .  in 0
 .  nf
 .  di @@c
 ..
 @endExample
 @Example
-.\" macro .)c terminates centering mode
+.\" Macro .)c terminates centering mode.
 .de )c
 .  br
 .  ev
 .  di
 .  nr @@s (((\n[.t]u - \n[dn]u) / 2u) - 1v)
 .  sp \n[@@s]u
 .  ce 1000
 .  @@c
 .  ce 0
 .  sp \n[@@s]u
 .  br
 .  fi
 .  rr @@s
 .  rm @@c
 ..
-.\" End of macro definitions, restore escape mechanism
+.\" End of macro definitions; restore escape character.
 .ec
 @endExample
 @endDefreg
 
-@DefescList {\\!, , , }
+@DefescList {\\!, , anything, }
 @DefescListEndx {\\?, , anything, \\?}
 @cindex transparent output (@code{\!}, @code{\?})
 @cindex output, transparent (@code{\!}, @code{\?})
-Prevent requests, macros, and escapes from being interpreted when read
-into a diversion.  Both escapes take the given text and
-@dfn{transparently} embed it into the diversion.  This is useful for
-macros that shouldn't be invoked until the diverted text is actually
-output.
-
-The @code{\!} escape transparently embeds text up to and including the
-end of the line.  The @code{\?} escape transparently embeds text until
-the next occurrence of the @code{\?} escape.  Example:
-
-@Example
-\?@var{anything}\?
-@endExample
-
-@cindex @code{\?}, and copy-in mode
-@cindex copy-in mode, and @code{\?}
-@cindex mode, copy-in, and @code{\?}
-@cindex @code{\!}, and copy-in mode
-@cindex copy-in mode, and @code{\!}
-@cindex mode, copy-in, and @code{\!}
+@dfn{Transparently} embed @var{anything} into the current diversion,
+preventing requests, macro calls, and escape sequences from being
+interpreted when read into a diversion.  This is useful for preventing
+them from taking effect until the diverted text is actually output.  The
+@code{\!} escape sequence transparently embeds input up to and including
+the end of the line.  The @code{\?} escape sequence transparently embeds
+input until its own next occurrence.
+
+@cindex @code{\?}, and copy mode
+@cindex copy mode, and @code{\?}
+@cindex mode, copy, and @code{\?}
+@cindex @code{\!}, and copy mode
+@cindex copy mode, and @code{\!}
+@cindex mode, copy, and @code{\!}
 @noindent
-@var{anything} may not contain newlines; use @code{\!}  to embed
-newlines in a diversion.  The escape sequence @code{\?} is also
+@var{anything} may not contain newlines; use @code{\!} by itself to
+embed newlines in a diversion.  The escape sequence @code{\?} is also
 recognized in copy mode and turned into a single internal code; it is
 this code that terminates @var{anything}.  Thus the following example
 prints@tie{}4.
 
 @Example
 .nr x 1
 .nf
 .di d
 \?\\?\\\\?\\\\\\\\nx\\\\?\\?\?
 .di
 .nr x 2
 .di e
 .d
 .di
 .nr x 3
 .di f
 .e
 .di
 .nr x 4
 .f
 @endExample
 
-Both escapes read the data in copy mode.
+Both escape sequences read the data in copy mode.
 
 @cindex @code{\!}, in top-level diversion
 @cindex top-level diversion, and @code{\!}
 @cindex diversion, top-level, and @code{\!}
 If @code{\!} is used in the top-level diversion, its argument is
-directly embedded into the @code{gtroff} intermediate output.  This can
-be used for example to control a postprocessor that processes the data
-before it is sent to the device driver.
+directly embedded into GNU @code{troff}'s intermediate output.  This can
+be used, for example, to control a postprocessor that processes the data
+before it is sent to an output driver.
 
 @cindex @code{\?}, in top-level diversion
 @cindex top-level diversion, and @code{\?}
 @cindex diversion, top-level, and @code{\?}
 The @code{\?} escape used in the top-level diversion produces no output
 at all; its argument is simply ignored.
 @endDefesc
 
 @cindex @code{\!}, and @code{output} request
 @cindex @code{output} request, and @code{\!}
-@cindex @code{output} request, and copy-in mode
-@cindex copy-in mode, and @code{output} request
-@cindex mode, copy-in, and @code{output} request
+@cindex @code{output} request, and copy mode
+@cindex copy mode, and @code{output} request
+@cindex mode, copy, and @code{output} request
 @Defreq {output, string}
-Emit @var{string} directly to the @code{gtroff} intermediate output
+Emit @var{string} directly to GNU @code{troff}'s intermediate output
 (subject to copy mode interpretation); this is similar to @code{\!} used
 at the top level.  An initial double quote in @var{string} is stripped
 off to allow initial blanks.
 
-This request can't be used before the first page has started -- if you
+This request can't be used before the first page has started---if you
 get an error, simply insert @code{.br} before the @code{output} request.
 
 Without argument, @code{output} is ignored.
 
 Use with caution!  It is normally only needed for mark-up used by a
 postprocessor that does something with the output before sending it to
 the output device, filtering out @var{string} again.
 @endDefreq
 
 @Defreq {asciify, div}
 @cindex unformatting diversions (@code{asciify})
 @cindex diversion, unformatting (@code{asciify})
 @cindex @code{trin} request, and @code{asciify}
-@dfn{Unformat} the diversion specified by @var{div} in such a way that
-@acronym{ASCII} characters, characters translated with the @code{trin}
-request, space characters, and some escape sequences that were formatted
-and diverted are treated like ordinary input characters when the
-diversion is reread.  It can be also used for gross hacks; for example,
-the following sets register@tie{}@code{n} to@tie{}1.
+@dfn{Unformat} the diversion @var{div} in a way such that Unicode basic
+Latin (@acronym{ASCII}) characters, characters translated with the
+@code{trin} request, space characters, and some escape sequences, that
+were formatted and diverted into @var{div} are treated like ordinary
+input characters when @var{div} is reread.  Doing so can be useful in
+conjunction with the @code{writem} request.  @code{asciify} can be also
+used for gross hacks; for example, the following sets
+register@tie{}@code{n} to@tie{}1.
 
 @Example
 .tr @@.
 .di x
 @@nr n 1
 .br
 .di
 .tr @@@@
 .asciify x
 .x
 @endExample
 
-Note that @code{asciify} cannot return all items in a diversion back
-to their source equivalent, nodes such as @code{\N[...]} will still
-remain as nodes, so the result cannot be guaranteed to be a pure string.
-
-@xref{Copy-in Mode}.
+@code{asciify} cannot return all items in a diversion to their source
+equivalent: nodes such as those produced by the @code{\N} escape
+sequence will remain nodes, so the result cannot be guaranteed to be a
+pure string.  @xref{Copy Mode}.  Glyph parameters such as the type face
+and size are not preserved; use @code{unformat} to achieve that.
 @endDefreq
 
 @Defreq {unformat, div}
-Like @code{asciify}, unformat the specified diversion.  However,
-@code{unformat} only unformats spaces and tabs between words.
-Unformatted tabs are treated as input tokens, and spaces are stretchable
-again.
-
-The vertical size of lines is not preserved; glyph information (font,
-font size, space width, etc.)@: is retained.
+Like @code{asciify}, unformat the diversion @var{div}.  However,
+@code{unformat} handles only tabs and spaces between words, the latter
+usually arising from spaces or newlines in the input.  Tabs are treated
+as input tokens, and spaces become adjustable again.  The vertical sizes
+of lines are not preserved, but glyph information (font, type size,
+space width, and so on) is retained.
 @endDefreq
 
+@codequotebacktick off
+@codequoteundirected off
+
 
 @c =====================================================================
 
 @node Environments, Suppressing output, Diversions, gtroff Reference


>   The Chapter "Diversions" in groff_tmac(5) contains a missing piece for
the
> "info" file.

Here is that material.


   Diversions
       Diversions can be used to implement quite advanced programming
       constructs.  They are comparable to pointers to large data
       structures in the C programming language, but their usage is
       quite different.

       In their simplest form, diversions are multi-line strings, but
       diversions get their power when used dynamically within macros.
       The (formatted) information stored in a diversion can be
       retrieved by calling the diversion just like a macro.

       Most of the problems arising with diversions can be avoided if
       you remember that diversions always store complete lines.  Using
       diversions when the line buffer has not been flushed produces
       strange results; not knowing this, many people get desperate
       about diversions.  To ensure that a diversion works, add line
       breaks at the right places.  To be safe, enclose everything that
       has to do with diversions within a pair of line breaks; for
       example, by explicitly using .br requests.  This rule should be
       applied to diversion definition, both inside and outside, and to
       all calls of diversions.  This is a bit of overkill, but it works
       nicely.

       (If you really need diversions which should ignore the current
       partial line, use environments to save the current partial line
       and/or use the .box request.)

       The most powerful feature using diversions is to start a
       diversion within a macro definition and end it within another
       macro.  Then everything between each call of this macro pair is
       stored within the diversion and can be manipulated from within
       the macros.


I don't care for much of this.  It opens with editorializing about advanced
usage and an analogy to pointers in C, both of which I think suggest to the
reader that a facility with diversion use will be difficult to acquire and
possibly unrewarding.

It is not tightly or concisely written.  Does it communicate any concrete
information about _groff_ that our Texinfo manual (now) does not?  I can't see
anything, and I am inclined to drop this subsection of groff_tmac(5) entirely,
as proposed in bug #61280.

Let me know what you think.  If I don't hear back, I will probably proceed as
above.

    _______________________________________________________

Reply to this item at:

  <https://savannah.gnu.org/bugs/?51072>

_______________________________________________
  Message sent via Savannah
  https://savannah.gnu.org/